1. Field of the Invention
The present invention generally relates to membrane switches and specifically relates to multiple pole momentary contact membrane switches having improved contacts.
2. Description of Prior Art
Existing membrane switches comprise a dielectric substrate, a flexible membrane which extends parallel to the substrate and a separator which is inserted between opposed surfaces of the membrane and the substrate. Circuit conductors and switch sites are provided on the opposed surfaces and frequently either the membrane or the substrate will be provided with an integral tail onto which the circuit conductors extend. The switch can then be connected to external circuits by means of a connector coupled to the end portions of the conductors on the tail.
The substrate and indeed the membrane can comprise a sheet of insulative material, such as Mylar with circuit patterns disposed thereon. Mylar can also be used as the spacer layer, although such spacer layers may also be formed by applying a printed or silk screened aperture pattern of a dielectric material to one or both of the substrate and membrane layers. The advantage of a printed or silk screened dielectric layer is that it can generally be made thinner than conventional spacer sheets. Silk screened layers can be on the order of from 0.001 to 0.002 inches thick while conventional spacer sheets are typically 0.003 to 0.005 inches thick.
Membrane switches have achieved wide acceptance for momentary contact switch applications. When the flexible plastic membrane which carries the movable contact is deformed, it moves towards a fixed contact on the substrate and, upon physically touching the fixed contact, the electrical switch connection is made. Upon release of the tactile pressure, the circuit is broken. While the above construction works well for single pole switches, there are a number of applications which require multi-pole switch contact, i.e. actuation of the switch resulting in the contacting of several circuits simultaneously. It is this multi-pole application which has caused problems for the conventional membrane switch.
Conventional multi-pole membrane switches have a spot-contact which is inherent in the membrane switch design. The sheet plastic membrane, which carries the movable contact, is flat only when the switch is not actuated. As soon as a force deforms the membrane towards the fixed contacts (whether through a tactile dome or a digit), the surface of the movable contact becomes curved. This curved surface touches the fixed surface and its contacts at a single spot at least initially. This then causes the curved surface to become flat at that single spot and with increasing pressure, the size of the flat portion grows to encompass the multiple contacts which may be placed on the fixed substrate. Accordingly, the multiple contacts may be made at different points in time resulting in a non-simultaneous interconnection.
Telephone equipment often uses the dual tone multi-frequency (DTMF) method for generating dialing tones. This method of tone generation requires two different audible frequency tones to be combined when a number key is pressed. A dual contact (two-pole) momentary membrane switch is a direct approach since one of each of the two frequencies can be continually present at each of two-pole contacts. These are mixed when the common movable contact touches both pole contacts. The failure to provide simultaneous making and breaking of the contacts can result in erroneous tone generation and misdialing of the telephone number. Obviously, if such a system is utilized as an input pad for a computer terminal, incorrect information can be supplied.